(1) Field of the Invention
The invention concerns a picture intensifier or amplifier tube and its use.
Such a tube produces a high-brilliancy light picture, which is formed on the tube output screen when the input screen is exposed to incident rays. Between these two screens a flow of electrons, emitted by a photocathode incorporated in the input screen, transfers the signal at each point from one end of the tube to the other.
(2) Description of the Prior Art
Such tubes are well known in the prior art, being used with incident X-rays, particularly for medical purposes.
In this case, they also act as converters, transforming incident X-rays into visible rays.
The output screen is generally an ordinary cathodoluminescent screen, which can emit light when bombarded by an electron flow. Consequently, it can work only in real-time, with the picture visible on the output screen only at the same moment as it is produced on the input screen. Traces may persist on the output screen, for periods that vary depending upon the arrangements adopted, but they are relatively short at the present stage of technical development of such screens.
Another type of screen exists, thin-layer electroluminescent screens, which can emit visible rays when excited by an electrical voltage applied between their two faces. This occurs above a characteristic threshold voltage, and for some of these screens the emission even persists for a certain time after the voltage has been reduced below this threshold. The drop in luminance then occurs with hysteresis, naturally causing memorization. A voltage can then memorize, display and erase a signal, as is usual with hysteresis phenomena.
This type of screen, like cathodoluminescent screens, is sensitive to electron bombardment, but it is used for a different purpose: the electron bombardment here adds its own action to the action of the applied voltage; it adds further voltage to the voltage applied to the input. Specifically, it makes it possible for the combined voltages to rise above the threshold voltage even when an input voltage less than this threshold is supplied to the screen. Generally, electron bombardment makes it possible to process the signal in various ways described in fuller detail below.
In the prior art, the usefulness of employing electroluminescent output screens has already been recognized in many types of electronic tubes, to obtain extended persistence of images, and thereby to memorize such pictures. Several devices with tubes equipped with such screens have been proposed, e.g., French Pat. No. 2,431,184, and U.S. Pat. No. 3,908,148.
However, all these developments concern cathode-ray tubes (CRT), tubes in which an electron beam scans the display screen to form a picture on it sequentially, point by point.
Such electroluminescent screens would be even more useful in tubes where the image on the output screen is formed by an electron flow which impacts the whole screen surface simultaneously, so that at any given moment, the screen is wholly "sprayed" by the electron beam. This spraying may be either constant during filming or may consist of pulses.
This is typically the case for radiological picture intensifiers (RPI), i.e., amplifiers used for medical purposes and light picture intensifiers (LPI), i.e., amplifiers used to obtain pictures under poor nocturnal lighting conditions.